PROJECT SUMMARY I am an analytical chemist committed to developing new methods to study how infant nutrition influences health trajectories in early life. In consultation with my mentors, I have developed a training plan to gain additional skills that will enable me to become an independent transdisciplinary researcher in Nutritional Epidemiology. The proposed research project will facilitate necessary training and develop a new biomarker that can quantify breast milk vs formula intake and composition, thereby overcoming a major limitation to the study of long term impacts of infant nutrition and lay the foundation for future grant applications. I have assembled an expert panel of mentors with complementary expertise, covering pediatrics, child development, epidemiology, nutrition, exposure biology and advanced statistics. Through mentorship, formal coursework and didactic seminars I will gain knowledge of children's health and nutrition and skills in exposure biology and advanced statistics. With these new skills, I can follow my long-term career goal of establishing a competitive, independently funded research program to study infant nutrition and associated long term impacts on health. While this study focuses on children's neurodevelopment, it is important to note that the knowledge and skills obtained will be applicable to multiple child health outcomes that may be linked to nutrition (e.g. obesity, immunological function, allergies and diabetes) and lays the foundation to later include prenatal nutrition as the biomarker can capture the prenatal period. Specifically, I will 1) gain comprehensive training in the application of biomarkers to study the association of chemical and dietary factors and health outcomes; 2) learn the measures used to assess neurodevelopment in children; 3) learn the role of nutrition in infant health; and 4) learn and gain experience with the statistical tools required to analyze complex multidimensional data. There is growing evidence that some adult diseases are programmed through infant diets but studies are hampered by the lack of a reliable, objective marker that can accurately reconstruct infant diet. The proposed research project will develop the first objective, temporal biomarker of infant nutrition and will demonstrate its application to study the effect of infant nutrition on children's neurodevelopment. This biomarker will reconstruct past diet objectively and quantitatively. We will use state-of-the-art analytical techniques to determine the distribution of multiple elements in children's teeth. Like trees, teeth contain growth rings that can be aged and the elemental content of each ring reflects the dietary intake at that time. Our preliminary results show that specific element signatures can be used to determine diet transitions from exclusive breastfeeding to infant formula. We will expand upon this work to develop a more robust index that can be used to determine the duration, dose and composition of breast milk or formula intake. We will apply the biomarker to study the effect of infant diet on neurodevelopment. This study is highly cost-effective as we will leverage the resources of an existing longitudinal birth cohort where all supporting data has been collected.